In recent years, significant advancements in performance as well as in size and weight reductions have been made in various devices such as video movie cameras, cassette tape recorders, communication equipment etc. These devices need a small-sized magnet, which is usually produced by machining a block of bonded or sintered magnet material.
To improve the performance of such devices, it is desirable to employ a magnet having a high maximum energy product. On the other hand, in small-sized magnet applications, it is also required that the magnet be easily machinable into a desired shape. Although sintered magnets have a large maximum energy product ranging up to 370 kJ/m.sup.3, they are very brittle and thus difficult to machine into a small size. Therefore, sintered magnets are unsuitable for use as small-sized magnets. However, bonded magnets have the advantage that they can be easily formed into a small size by machining, and thus most common millimeter-sized magnets are now of this type. However, this type of magnet has the disadvantage that the maximum energy product is as low as 40 to 120 kJ/m.sup.3 for mass-produced magnets and 170 kJ/m.sup.3 for Labaratory-produced magnets.
Cylindrical magnets having radial anisotropy for use in miniature motors, rotation sensors, or the like are produced by means of the in-magnetic-field formation technique or the extrusion technique. In the case of the in-magnetic-field formation technique, the inner diameter of the cylindrical magnet must be above a minimum limit so as to produce a magnetic field in a radial direction. At the present time, the practical minimum outer diameter of a magnet of this type is about 1 cm. When extrusion is employed, a mold having a minimize size is needed to ensure that the mold can withstand process pressures. Again, the current lower limit of the outer diameter of the magnet is about 1 cm. These magnets are further machined so as to obtain a good circular form with the dimensional accuracy required for particular applications. However, the above-described methods are unsuitable for producing cylindrical magnets having radial anisotropy and a size of about a millimeter or less.
In applications for micro-machines with a body size less than 1 cm.sup.3 to be used in examination and repair robots for industrial and medical uses, magnets having a very small size such as a few mm.sup.3 or less are required. However, such small-sized magnets cannot be produced by means of practical machining techniques.
One known technique for producing such magnets is physical vapor deposition such as by sputtering. This technique allows production of small-sized magnets with submicron accuracy. This technique allows control of various magnet characteristics, such as internal stress, crystallinity, and crystal orientation, by adjusting film deposition conditions. Utilizing various advantages of this technique, rare earth alloy-based thin film magnets have recently been developed. For example, Japanese Patent Laid-Open No. 4-99010 (1992) discloses a technique for producing a thin film magnet having a maximum energy product as large as 80 to 111 kJ/m.sup.3 by properly selecting the composition of Nd--(Fe, Co, Al)--B within a certain range and also by properly selecting substrate temperature and deposition rate.
To achieve a further size reduction while maintaining device performance, it is necessary to use a magnet having a higher maximum energy product than those of the bonded magnets which are now widely used in small-sized devices. However, the maximum energy product of the conventional thin film magnet is not greater than that of the bonded magnet.
Furthermore, in the case of cylindrical magnets having radial anisotropy for use in small-sized motors or small-sized rotation sensors, the magnet must be formed in a circular shape having less than about a 10 .mu.m deviation from an ideal circular shape and also having high radial dimensional accuracy of a similar order. In the conventional technique, as described above, a difficult machining process is required to achieve such high dimensional accuracy. The conventional technique has a further problem in that it is difficult to produce a cylindrical magnet having radial anisotropy with a size less than about a millimeter.